1. Field of the Invention
The invention relates to a ball bearing.
2. Description of the Related Art
Ball bearings involve a lower torque (smaller rotational resistance) than roller bearings such as tapered roller bearings. However, there has been a demand for a further reduction in the torque of ball bearings. As depicted in FIG. 7, a conventional ball bearing 90 has an inner ring 91, an outer ring 92, a plurality of balls 93, and an annular cage 94. The balls 93 are held at intervals in a circumferential direction by the cage 94. The cage 94 is guided (positioned) by coming into surface contact with the balls 93 (see, for example, Japanese Patent Application Publication No. 2014-70669 (JP 2014-70669 A). In the ball bearing 90 depicted in FIG. 7, sealing apparatuses 95 are provided on axially opposite sides of the bearing so as to avoid external leakage of grease present between the inner ring 91 and the outer ring 92.
In the conventional ball bearing 90, a slipping velocity between the cage 94 and each of the balls 93 is very high, leading to a very high shear velocity of the grease present between the cage 94 and the ball 93. This increases viscous resistance of the grease, disadvantageously resulting in an increased rotational torque of the ball bearing and a shortened life of the grease.